First of all, its too big to be an avatar. You would need to use something like photoshop to scale it down.
Before you can use this avatar , you should ask permission from the website owner ( although most people don't bother ).
I would first get permission to use it, then scale it down to a decent size, then just upload it to the forum/s you wish to use it on.
Some of the forums you are using may not allow files to be uploaded. If that is the case, you will need to point the avatar option in your control panel to point to the image on the web. Webmasters don't like this method, since you are illegally using their bandwidth by doing this, since everytime the avatar is displayed it would be called from their web server. Although you could save the file to your harddrive and upload to a free hosting service , then point the avatar url to this location.
Its pretty simple really, but i am sorry - i am not the best person in explaining things.
Good Luck
